I have to do some UI testings on iPhone X, but I don't have a device or even a Mac. Is there any way to test an iPhone app on an iPhone X simulator on a Windows environment.
Browserstack have recently added iPhone X to their real device list.
It'll cost you, and the price will vary depending on where you are and what you want out of it, but you can spin up an iPhone X (among loads of other phones) within 60 seconds.
As someone already mentioned there are a number of devices or testing services available and most have either a free tier of usage or some amount of time where the service is free. These work in the short term but can be a bit slow:
Realistically if you are doing any any serious or long term testing you'll want to get your hands on a real device. This might mean purchasing directly, renting one (google search) or finding a device lab in your area where you can rent time / space.